Currently, success has been found by relieving back pain using procedures which implant devices or bone which allow adjacent vertebrae to be fused together across a disk space. These devices generally have apertures and a central cavity, and are packed with bone chips or other bone growth inducing substances. Upon implantation between adjacent vertebrae, the devices relieve the back pain due to dysfunctional disks or due to other reasons, and at the same time provide a site for fusion. Bone growth from the upper and lower vertebrae proceeds through the apertures of the implanted device, uniting with the bone growth inducing substance so that the upper and lower vertebrae fuse together through and around the implant.
Alternatively, an implant made entirely of bone can be implanted to create an interbody fusion between vertebrae.
A number of procedures are used to accomplish such implants. These procedures include approaches to the spinal column from anterior, posterior, and lateral directions, just to name a few.
Prior to the implantation of a device, the implant site must be prepared. Prior art procedures dictate that the site including adjacent vertebrae and the disk located therebetween, must be exposed. As there are major blood vessels and nerves which are protected by and surround the spinal column, care must be taken so that these are not disturbed. Accordingly, after the site is exposed, sheathing devices are located adjacent to the site. The site preparation procedure can then be carried out inside the sheathing devices in order to protect the adjacent blood vessels and nerves. Historically, these sheathing devices are cylindrical in nature with a front end having prongs which project therefrom. The sheathing device is placed adjacent the upper and lower vertebrae, and spans the disk space. The sheathing device is then tapped so that the projecting prongs engage with the outside faces of the upper and lower vertebrae in order to stabilize the sheathing device relative to the vertebrae. After this is accomplished, a drill bit can be inserted in the sheathing device in order to drill out and remove the disk material and both cortical and cancellous bone. The cortical bone provides the hard outer surface of the vertebral body while the cancellous bone is internal and is softer and porous and provides the passages for the blood supply which nourishes the bone.
Such a site preparation technique has been used successfully in the past, however, it is still desirable to improve upon this procedure and make it safer and easier to perform. For example, with the prior art sheathing device, a purchase must be obtained on the anterior, lateral or posterior facing sides of the upper and lower vertebral bone with the prongs extending from the sheathing device. Accordingly, it is not always possible to obtain and maintain the desired alignment of the sheathing device as the operation is carried out. Also as a sheathing device is needed to encase the drill, the diameter of this instrumentation may be a little larger than desired. Further, by using a drilling operation even though the disk space can be cleared out using appropriate cutting instruments, prior to the drilling operation, the drill bit can still become clogged with fibrous disk material and require cleaning prior to the completion of the site preparation.
Further, in such a procedure there is no mechanism for stopping the forward movement of the drill. The physician relies on indicia marked on the drill or other depth gaging devices for ensuring that the drill does not penetrate too far or remove too much material.
Against this background, there is a continuing need to improve the process of preparing a site for implantation.